1. Field of the Invention
The present invention relates to fluid suspension systems for vehicles and, more particularly, to height control valves for use in such fluid suspension systems.
2. Description of the Prior Art
Air suspension systems are commonly used in vehicles for maintaining the frame above the axles by a predetermined spacing. Air springs are often employed in such suspension systems to support the frame wherein supplying air to the air spring raises the vehicle frame, while exhausting air from the air spring lowers the vehicle frame. Most air suspension systems incorporate height control valves for maintaining a desired spacing or height of the frame relative to an axle of the vehicle. The height control valve senses when the spacing between the frame and axle is greater or less than the desired value and, accordingly, adjusts the air flow to and from the air spring.
Height control valves are typically mounted on the vehicle frame and include a position arm which is operably connected to the axle through a linkage. As the distance between the vehicle frame and the axle varies, the linkage causes the position arm to rotate a shaft inside the height control valve which selectively permits air to be supplied to or exhausted from the air spring.
A conventional height control valve has three ports: an air spring port connected to the air spring, an inlet port connected to a source of pressurized air, and an exhaust port open to the atmosphere. To increase the spacing between the vehicle frame and the axle, the height control valve permits fluid communication between the air spring port and the inlet port thereby allowing air to flow from the pressurized air source to the air spring. To decrease the spacing of the vehicle frame relative to the axle, the height control valve permits fluid communication between the air spring port and the exhaust port, thereby allowing air to flow out of the air spring to atmosphere. When the vehicle is at the desired height relative to the axle, the valve is in a neutral position in which neither the inlet port nor exhaust port communicates with the air spring.
While the prior art height control valves have been effective in maintaining a substantially constant average height of the vehicle frame relative to an axle, they have failed to provide effective means for preventing operation of the valve in response to sudden and momentary variations in spacing between the frame and axle as may result from the vehicle passing over surface irregularities. Attempts by some prior art valves to correct for such minor irregularities or disturbances results in continuous valve operation in order to find the desired height of the frame, thereby increasing air consumption and draining the source of pressurized air. While some prior art valves provide for a time delay during initial periods of spacing variations between the frame and axle so as to prevent admission of air to or discharge of air from the air spring in response to relatively minor differential spacings between the frame and axle, these valves fail to provide efficient and accurate control of relative frame height during vehicle loading and unloading. More particularly, the time delay of such prior art valves prevent the rapid and accurate admission of air to or discharge of air from the air spring in response to differential spacing between the frame and axle resulting from vehicle loading or unloading.
Accordingly, there is a need for a height control valve which effectively restricts fluid flow to and from an air spring in response to slight variations in height or spacing between a vehicle frame and axle while also providing rapid response to and accurate control of spacing variations resulting from vehicle loading and unloading.
During operation, it is often desirable to rapidly exhaust the air from within the air springs to atmosphere thereby quickly lowering the vehicle frame relative to the axle. For example, on trucks comprising tractors pulling trailers, the air is often "dumped" or completely exhausted from the tractor air springs, thereby lowering the tractor frame relative to the trailer. Prior to exhausting the air, the operator typically lowers the front dolly legs on the trailer until they touch the ground. After dumping the air from the air springs and lowering the tractor, the full weight of the trailer is supported by the dolly legs and the tractor is free to separate from the trailer.
Likewise, it is also often desirable to quickly "dump" or completely exhaust the air from the air springs of a trailer to quickly lower the trailer frame relative to the axles. For example, during loading of the trailer it is preferable to have the trailer frame in a fixed position relative a loading platform or dock. By dumping the air from the air springs, the frame is lowered until it is supported on a hard stop to thereby prevent floating or creeping of the trailer away from the loading platform or dock.
Dump valves have been utilized in the prior art to quickly exhaust the air contained within air springs to atmosphere. However, such dump valves are typically independent devices which must be separately mounted to the vehicle frame and plumbed between the air spring and the height control valve. While attempts have been made at incorporating dump valves within height control valves, the resulting valves have comprised complicated sealing arrangements defining complex air flow paths through the height control valve such that it takes a relatively long time to completely exhaust air from the air spring.
Furthermore, the prior art dump valves typically provide for activation only by the application of increased air pressure to a positive pilot port. Since conventional tractors have independent air compressors, increased pilot pressure may be utilized to activate the dump valve and exhaust the air spring supporting a tractor frame. However, conventional trailers do not include separate sources of pressurized air and therefore an independent source of pressurized fluid must be provided to activate the dump valve and exhaust the air spring supporting a trailer frame.
Accordingly there is a need for a height control valve having a simple and efficient integral dump valve. Further, there is a need for such a dump valve which may be activated by a decrease in fluid pressure applied to a negative pilot port.